In various electric wires such as insulted wires, shielded wires and insulated cables, the conductor or the outer coating is insulation-coated with an insulating material. As the insulating material for wires such as insulated wires or insulated cables for use for inner wiring in electronic appliances, generally used are a polyvinyl chloride resin and a polyolefin resin composition containing a flame retardant. The polyolefin resin is typically an ethylene copolymer such as ethylene-vinyl acetate copolymer and ethylene-ethyl acrylate copolymer. As the flame retardant, generally used is a halogen flame retardant containing a bromine atom or a chlorine atom in the molecule. A bromine flame retardant having a bromine atom in the molecule, which is one of a type of the halogen flame retardant, has a high flame-retarding effect, and in general, it attains flame retardation, taking advantage of the synergistic effect with antimony trioxide combined with it. The bromine flame retardant is highly effective, when combined with a phosphorus compound.
However, when wires coated for insulation with a coating material such as a polyvinyl chloride resin or a polyolefin resin containing a halogen flame retardant are discarded, the plasticizer, the heavy metal stabilizer and the phosphorus compound contained in the coating material may dissolve out to pollute the environment. In addition, when the wires insulation-coated with such a coating material are incinerated, the halogen compound contained in the coating material may generate corrosive gases and dioxins.
For good approach to the increasing requirement for environmental load reduction these days, halogen-free wires have been developed, which use a coating material not containing a polyvinyl chloride resin and a halogen flame retardant. On the other hand, in general, wires such as insulated wires and insulated cables for use for inner wiring in electronic appliances are required to have various characteristics satisfying the UL (Underwriters Laboratories Inc.) standard. The UL standard defines in detail various characteristics that products must satisfy, such as flame retardancy, thermal deformation, low-temperature characteristics, and initial and thermally-aged tensile characteristics of coating materials. Regarding the flame retardancy of those, products must pass a vertical wire flame test called a VW-1 test, and this is one of the severest requirements of the UL standard.
In general, as a coating material for halogen-free wires, there is used a flame-retardant resin composition including a polyolefin resin with a metal hydroxide (also called as a metal hydrate) such as magnesium hydroxide or aluminium hydroxide added thereto. However, the flame-retarding effect of metal hydroxide is low as compared with a halogen flame retardant, and therefore, for making it to pass the vertical wire flame test VW-1, a large amount of metal hydroxide must be added to polyolefin resin. As a result, the tensile characteristics (such as tensile strength and tensile elongation at break) and the heat deformation resistance of coating materials may greatly worsen.
When a coating layer of a resin composition including a polyolefin resin and a metal hydroxide is crosslinked through irradiation with ionizing radiations such as accelerated electron beams, then its tensile characteristics and heat deformation resistance may be improved. However, the halogen-free flame-retardant resin composition mentioned above is more expensive than polyvinyl chloride resin and, in addition, it has another drawback in that its production cost further increases since an expensive irradiation apparatus is necessary for irradiation with ionizing radiations. Accordingly, it is desired to develop a halogen-free wire capable of satisfying the UL standard even in the absence of crosslinking treatment.
Heretofore, as a halogen-free flame retardant resin composition, there is proposed a transmission line-coating resin composition including a resin component containing an ethylene copolymer and a polyester elastomer and a large amount of a metal hydride added thereto (Patent Document 1). As the ethylene copolymer, used is an ethylene-vinyl acetate copolymer having a vinyl acetate content of 25 to 85% by mass. However, the polyester elastomer-containing flame-retardant resin composition disclosed in Patent Document 1 is not always satisfactory in point of its flame retardancy and insulation resistance and, in particular, its pass rate in the vertical wire flame test VW-1 is not high.
A flame-retardant resin composition is proposed, as prepared by melt-kneading a resin component containing an ethylene copolymer and a polyester-type and/or polyether-type segment-having thermoplastic resin, with an organic peroxide and a silane coupling agent-processed metal hydrate (Patent Document 2).
Patent Document 2 shows, for example, a thermoplastic polyester elastomer, a thermoplastic polyurethane elastomer and a thermoplastic polyamide elastomer, as the polyester-type and/or polyether-type segment-having thermoplastic resin. However, even though a metal hydrate is added to the resin composition that contains a thermoplastic polyurethane elastomer (DIC Bayer Polymer's trade name, “T-8180N”) and an ethylene-vinyl acetate copolymer having a vinyl acetate content of 41% by weight (Mitsui DuPont's trade name, “Evaflex EV40LX”) shown in Examples of Patent Document 2, it is difficult to obtain a resin composition having excellent flame retardancy, mechanical properties, heat resistance, heat aging resistance and heat deformation resistance, and in particular, it is extremely difficult to obtain a resin composition showing high flame retardancy that passes the vertical wire flame test VW-1.    Patent Document 1: JP 2004-10840 A    Patent Document 2: JP 2004-51903 A